1. Field of the Invention
The present invention relates to a photodiode, and a method for its fabrication, which can be fabricated in an integrated circuit and which has a reduced series resistance.
2. Description of the Prior Art
In various integrated circuit arrangements, e.g. in optocouplers and electronic relays, photodiodes are employed as integrated energy and signal sources. For this application, the photodiodes are to have a high off-load voltage, a high short-circuit current and a maximum power output. The power provided by the photodiode does depend on the internal resistance of the photodiode. The internal resistance is formed by the series resistance which results from the sheet resistances of the diffusion zones of the photodiode, the contact resistances of the metallic coatings of the photodiode, and the conduction resistances of the metallic coatings (see, e.g., S. M. Sze, "Physics of Semiconductor Devices", Wiley, New York, Ch. 14.3.2).
Photodiodes which are fabricated as individual components are provided, in order to reduce the series resistance and to increase the photon yield, with back contacts and special geometries to reflect the light; e.g., V trench patterns (see, e.g., S. M. Sze "Physics of Semiconductor Devices", Wiley, New York, Ch. 14.3.4).
Photodiodes which are fabricated in a state integrated in a substrate cannot be provided with back contacts or reflection patterns. Photodiodes which are integrated in a substrate have a p-n junction near the top of the component, which separates the electrons and holes generated in the silicon by photons. The p-doped and n-doped zones are electrically connected in this arrangement via narrow, highly doped n-doped and p-doped strips, respectively. The highly doped strips have a high internal resistance. The sheet resistance of the conduction is reduced by means of a metallization layer which has contacts over the entire length of the highly doped strips. At the same time, part of the photosensitive area of the photodiode is covered, so that the photon yield is reduced.